The Effect Of Strain-softening Behavior On Bearing Capacity Of Plate Anchors In Clay

Many constructions require foundations to provide uplift resistance to withstand natural loads, such as wind loading on transmission towers, wave action on offshore structures and buoyancy forces on buried pipelines. As easily and economically assembled, plate anchors have been increasingly used as foundations in floating platforms in offshore gas and oil exploitation engineering. Under wind, wave and tides loading, plate anchor foundations are subjected not only static loading but also cyclic loading with relatively long period. Plate anchors in clay transfer the loading into the clay around and then get the capacity of uplift resistance, as the cyclic number grows, the plastic shear strain of clay increases as a result of which the shear strength of clay decreases. The embedded depth and overburden pressure affect the ultimate bearing capacity of plate anchors and responding soil flowing mechanisms around; and thus the softening mechanisms of plate anchor capacities for various cases worth further more research. In this paper, numerical analysis and model tests are adopted to study the effect strain-softening behavior on the bearing capacity of plate anchors.Firstly, the ultimate bearing capacity factors in clay with various cases were calculated by a finite difference program named FLAC and the effect of overburden pressure on soil flowing mechanism was studied around shallowly and deeply embedded anchors respectively. Generally speaking in terms of the ultimate limit condition, shallowly embedded anchors have relative low stress level and thus the shear bonds develop vertically from the two edges of anchors to the soil surface, showing a "strip" shape flowing mechanism; For the deeply embedded anchors at low stress level, the shear bonds develop from the edges of anchors obliquely at first and then almost vertically to the soil surface, named as a "U" shape mechanism; the soil mechanisms demonstrate a "two rings" shape for the deeply embedded anchors with high stress level. Strain-softening model was adopted in FLAC to analyze the effect of residual strength parameters δrem and referring shear strain ξ95on the strain-softening behavior of bearing capacity of plate anchors in monotonic cases. Different formulae are put forward to calculate the pullout capacity of plate anchors taking consideration of the strain-softening behavior of clay shear strength.Afterwards, model tests of anchors subjected to cyclic loading in clay were conducted, and the effect of strain-softening behavior of clay around on bearing capacities of plate anchors was studied. During the tests T-bar or Cone-bar was used to decide the clay shear strength along the depth. During the plate anchor tests at low stress level, PIV analysis was performed to analyze the soil flowing mechanism, demonstrating that the shear bonds developed from the two edges of the plate anchor to the soil surface with the hole occurring under the anchor; the subsequent bearing capacity decreased significantly with the anchor moving up and down the hole. A fire hose was paved along the soil surface with air pressure added in to increase the stress level and the cyclic uplifting model tests were performed, of which the results show that the hysteresis cycles in tests at high stress level were more evident than those at low stress level. The uplifting bearing capacity equals to the pressing down capacity, as the cyclic number grows the bearing capacity decrease, which is the due to the strain-softening of clay shear strength along the shear bonds.